Smoke and heat alarms are extensively used in domestic dwellings, motels, hotels, hospitals, old people's homes, and in general commercial premises. Most of these devices incorporate internal batteries and are generally are of two main types, namely:
(1) single supply smoke and heat alarms powered only by their internal batteries and PA0 (2) dual supply smoke and heat alarms where the alarm devices are powered by an external primary power supply with internal batteries as the standby power source. PA0 (1) If no replacement battery is available at the time when the smoke or heat alarm starts emitting the low battery warning, occupants of the protected premises have to endure the inconvenience of the audible warning until such time as they procure themselves of a new battery and carry out the battery replacement. The audible warning is particularly inconvenient if the alarm device is installed in a bedroom and starts to emit the low battery audible warning signal in the middle of the night with the result that the occupants of the bedroom, or any other bedroom in the area, cannot go back to sleep. PA0 (2) It is known that often occupants, of dwellings particularly, resort to drastic unsafe practices in order to eliminate the inconvenience of the low battery warning until such time as a replacement battery is available. These unsafe practices are: PA0 (a) the complete removal or disconnection of internal batteries of single supply smoke and heat alarms, and PA0 (b) the disconnection/turning off of both the primary and standby power sources of dual supply smoke and heat alarms. PA0 (1) as the smoke or heat alarm battery voltage under self-test mode approaches the low battery threshold voltage of the alarm device, a battery voltage is reached when the low battery pre-alarm warning is provided. The low battery pre-alarm warning may then be silenced if a replacement battery is not immediately available, with the result that PA0 (2) the occupants of the protected premises receive prior warning, several hours or days in advance, of an impending low battery condition. During this time the occupants can procure themselves of a new battery to replace the one in use without having to resort to the unsafe practices of battery removal or disconnection/turning off of power supplies, or suffer the inconvenience of an ongoing audible warning being emitted. The low battery pre-alarm voltage threshold value is normally a few tenths of a volt higher than the low battery voltage threshold value of the smoke or heat alarm.
It is a characteristic of the above mentioned two types of smoke and heat alarms that at regular intervals, normally not exceeding 60 seconds, the alarm device enters into a self-test mode when a current which is much higher than the quiescent current is briefly drawn from the smoke or heat alarm battery. The electronic circuitry of the alarm device then detects whether the battery voltage in self-test mode is above a certain threshold value. If the battery voltage is detected to be below the low battery voltage threshold value, normally around seven and a half volts, the alarm device activates an internal circuit to produce an audible warning indicating a low or depleted battery requiring replacement. For safety reasons, currently manufactured smoke or heat alarms do not have provisions for the low battery warning to be silenced.
Although the low battery warning is an important safety feature of the smoke and heat alarms described above, as the warning cannot be silenced, a situation very often arises where the low battery audible warning causes major difficulties/disadvantages as exemplified below:
Both the above practices may result in potentially dangerous situations where the fire detection and warning functions of the alarm devices are lost.
It is the object of the present invention to provide a new facility to smoke and heat alarms which overcomes the above difficulties/disadvantages without affecting the functionality and the operation of these alarm devices, including the operation of the low battery warning function currently available.
According to the present invention, and in addition to the smoke and heat alarm low battery warning, a low battery pre-alarm warning is provided, which can be silenced, and which operates at a threshold voltage of value slightly higher than the smoke or heat alarm low battery threshold voltage, the operation of the low battery pre-alarm warning being characterised in that:
Also according to this invention, should the occupants of the protected premises decide to silence the low battery pre-alarm warning because a new battery is not immediately available, and then they fail to replace the battery in use, a point is reached when the battery is further depleted so that its voltage in self-test mode falls further and reaches the low battery voltage threshold value of the alarm device. The latter then resumes the emission of a warning to indicate and warn of a low battery condition. For safety reason, this warning cannot be silenced.